This invention relates to marine structures and to methods of constructing and erecting them.
Marine structures are used as platforms for drilling undersea oil wells and as oil or gas production platforms, but they can be used for other purposes such as lighthouses, storage tanks and single point moorings.
The problems of constructing offshore marine structures are known to increase with the water depths at the site, particularly at water depths exceeding 6-700 feet, and these problems are further aggravated when the site is exposed to winds and waves calling for bigger structures and leaving less time for their construction offshore. It is therefore necessary to construct such structures as far as possible at an inshore base, transport them to site when they are as near as possible to completion and devise methods for installing them safely on site in the shortest possible time with minimum interference from winds and waves.
In the past, such structures were constructed as prefabricated tubular steel space frames which were secured to the sea bed by pile driving, relying on very heavy floating equipment for the pile driving and the subsequent installation of the deck structure and all the drilling and production equipment well above the highest wave crest. In deep and exposed water this offshore installation period may become so long that there is not enough good weather time for the operation.
The present trend is to construct such structures as large floating concrete bases, compartmented in various ways, from which a number of vertical towers rise to such a height that they can reach above the highest wave crest and support the required deck structure and deck installations. It is possible to design structures along those lines which can float safely in a vertical position and remain stable during all stages of floatation, towing to the site and sinking to the sea bed. It is also possible to found such structures directly upon the sea bed quite quickly and without too much dependence upon winds and waves. To some extent this approach to offshore constructions overcomes the problems created by deep and exposed waters, but it still suffers from some distinct disadvantages.
The criteria for floating stability are however in direct conflict with the criteria for attracting minimum wave forces when resting on the sea bed and in consequence make the structures heavier and bulkier than the permanent conditions on the sea bed require. The structures furthermore require deep water for their construction and they become more economic the deeper the construction site is, since the conflict between floating and fixed stability becomes less acute. Very few suitable sites are available in areas such as surround Great Britain where that type of structure can be constructed competitively and the penalty for installing deck load on the structure before towing to site is very heavy insofar as it calls for even deeper water for construction or increases the size of the structure disproportionately. This again causes increased wave loads on the structure and increased foundation stresses which may become critical since structures of this nature for their safety are very dependent upon the strength of the upper soil strata and those strata often are quite weak. This limits the use of this type of structure -- and this limitation is further aggravated by the fact that the results of detailed soil surveys of the site in question quite frequently only become available a long time after the demand for the structure has been established. But since such soil information is required at the very beginning of the design and construction of the platform it imposes on the designer the strain of having to design a structure for rather ill-defined foundation conditions, and on the contractor of having to start construction of a not yet fully designed structure -- causing ultimately a most unwelcome delay in the delivery of the platform.
The main object of the present invention is to provide a marine structure in which the aforesaid problems have been greatly reduced and the structures consequently have become correspondingly easier and cheaper to design, construct and finally install in the sea bed. The structure in accordance with the present invention is governed by its floating stability and its required strength for temporary conditions, e.g. during erection being in harmony with permanent environmental and operational conditions. It does not require deep water for its construction and plenty of suitable construction sites can be found in most countries including Great Britain. The main purpose of the structure is to support heavy deck loads above the crest of the waves and such heavy loads can be installed before tow-out without a heavy structural penalty. It does not call for a deep water installation site, the operation is well within the capacity of ordinary floating crane capacity and it has no bearing on the final structural design, but it does require the introduction of additional temporary buoyancy elements. The structure in accordance with the present invention does however lend itself naturally to having the deck structure, complete with all the required drilling and production facilities, installed after tow-out since the basic structure, quite naturally, after the final founding in the sea bed can be stopped at a suitable depth below the water surface, and the superstructure in the shape of a conventional "jack-up" platform can be floated into position, landed on the three columns and jacked up.
Futhermore, the proposed method of foundation makes the structure independent of the sea bed topography and less dependent upon the weak upper soil strata and finally the most important feature, the proposed construction method permits the supporting feet to be constructed last and thus provides time for the design of the feet to be based upon up-to-date soil investigations without causing delays to the construction and therefore ultimately producing a quicker delivery of the structure.
According to the present invention a marine structure comprises three substantially vertical columns tied permanently together by bracings at their lower and upper portions with cross bracings within their lengths to form a rigid tower and feet at their lower ends to be sunk into the sea bed with means for excavating sea bed soil material inside the feet.
Preferably the columns are braced together at their top portions and lower portions by horizontal braces, the lower brace being adjacent to the sea bed in the erected structure and the upper brace being disposed at the upper end of the structure above the highest wave crest possible at the site, forming part of the deck structure.
The bracings within the lengths of the columns may be formed in sets fixed between adjacent columns, each set comprising two crossed members lending themselves for prefabrication horizontally before being secured to the columns.
Each column preferably has a conical hollow foot open at its lower end and having access means through which means for excavation may be passed to excavate the sea bed beneath the foot, as well as for inspection, sampling and testing before backfilling takes place. The hollow space in each foot is designed to provide buoyancy during construction and tow-out and to reduce the ground pressure underneath the foot if required.
Each column is a floatable structure with means by which it can be filled with ballast, e.g. water, to sink the structure in the water. Buoyancy tanks may also be attached temporarily to support the upper end of the floating columns and the required superstructure attached to the columns. The buoyancy tanks may be capable of ballasting and deballasting as with water to assist in maneuvering and supporting the structure when floating and sinking it to the sea bed.
From another aspect of the invention a method of constructing a marine structure according to the invention comprises making three hollow columns, bracing the columns together at their end portions and between their end portions to form a horizontally disposed rigid buoyant structure such that when erected on the sea bed no horizontal bracing members are in the critical wave zone, assembling hollow feet on the columns, floating the structure to the site with the columns substantially horizontal, ballasting one end portion of the columns to upend the structure into a substantially vertical position and to sink the structure onto the sea bed, and excavating the sea bed within the feet to enable the structure to be seated firmly in the upright position at a suitable foundation depth in the sea bed.